1. Field of the Invention
The present invention relates to a novel material. In particular, the present invention relates to a material which is preferably used in a light emitting element in which an organic compound is used in at least one part. In addition, the present invention relates to a light emitting element, a light emitting device, and an electronic appliance which include the material.
2. Description of the Related Art
Development of a light emitting device using a light emitting element which includes an organic compound containing layer between a pair of electrodes and emits light when current flows between the electrodes has been advanced. Such a light emitting device has the advantage of being thin and lightweight, compared with other display devices at present which are called thin display devices. Such a light emitting device also has high visibility since it is a self-light emitting element, and has fast response speed. Therefore, this kind of light emitting device has been actively developed as a next-generation display device, and has partly come into practical use.
A light emitting mechanism of the light emitting element is described below. When a voltage is applied between a pair of electrodes, electrons injected from a cathode and holes injected from an anode recombine with each other to form molecular excitons in a light emitting layer included in an organic compound containing layer. The molecular exciton releases energy when returning to a ground state, so that light is emitted. Singlet excitation and triplet excitation are known as excited states, and it is thought that light emission can be achieved through each of the excited states.
The organic compound containing layer provided between the electrodes may have either a single-layer structure including one light emitting layer or a stacked-layer structure including layers having different functions from each other; however, the latter, a stacked-layer structure of a function-separated type, is often employed. As an example of such a stacked-layer structure of a function-separated type, a structure where a hole injection layer, a hole transport layer, a light emitting layer, and an electron injection layer are sequentially stacked over an electrode serving as an anode is typical, and each layer is formed of a material specific to each function. Note that a layer having two or more kinds of these functions such as a layer having both functions of a light emitting layer and an electron transport layer or a layer having another function such as a carrier blocking layer may be used.
Among the foregoing functional layers, a light emitting layer can be roughly classified into two types according to its structure. One is a structure in which a light emitting layer is a single film of a light emitting substance. The other is a structure in which a light emitting layer is formed by dispersing a light emitting substance in a host material. The latter structure is advantageous since in the latter structure, a light emitting substance can be selected without depending on a crystalline property and a film forming property thereof, and concentration quenching is hardly caused.
In the case of using such a light emitting layer having a structure in which a light emitting substance is dispersed in a host material, the host material needs to have a larger energy gap than the light emitting substance. This is an important requirement aiming at prevention of changes in light emission efficiency and light emission color due to excitation energy of an excited light emitting substance moving to the host material, and aiming at improvement in light emission efficiency due to the excitation energy moving smoothly from the excited host material to the light emitting substance.
A color of light emitted from a light emitting substance depends on an energy gap of the light emitting substance. The larger energy gap a light emitting substance has, the shorter wavelength of light is emitted. Therefore, a host material used with a light emitting substance exhibiting blue light emission needs to have a very large energy gap. However, there have not been many of such materials yet (for example, see Patent Document 1). Further, a host material used with a light emitting substance exhibiting violet or ultraviolet light emission is required to have an even larger energy gap.
When a light emitting element is manufactured using the foregoing stacked-layer structure of a function-separated type, each functional layer is formed of a material suitable for each function. To obtain a high-performance light emitting element, each functional layer thereof is required to have excellent characteristics in every aspect. However, an electron transporting material for forming an electron transport layer has been less reported compared to a hole transport material for forming a hole transporting layer, and there has been a delay in development of the electron transporting material under the present condition.
For example, tris(8-quinolinolato)aluminum (abbr.: Alq3) which is widely used as an electron transporting material has an excellent electron transporting property and reliability. However, since its emitting color is green, it is understood that an energy gap of Alq3 is small. Therefore, in a light emitting element which emits light with wavelength of blue or shorter, Alq3 is difficult to be used as an electron transport layer being in contact with a light emitting layer. This is because when a light emitting region in a light emitting layer is closer to an electron transport layer side, excitation energy of a light emitting substance and a host material moves to the electron transport layer side which has a small energy gap.
To avoid such a problem, it is efficient to use an electron transporting material as a host material and to set a light emitting region in a light emitting layer closer to an hole transport layer side. However, as described above, an electron transporting material has been less reported and there are a very small number of electron transporting materials which can be used as a host material in a light emitting element which emits light with wavelength of blue or shorter.    [Patent Document 1] Japanese Published Patent Application No. 2005-132820